EXSS is Exercising Science Solutions for Public Impact

Individuals who suffer anterior cruciate ligament (ACL) injury are at a greater risk of developing osteoarthritis (OA). Though ACL reconstruction surgery restores joint stability, it does not reduce the risk of developing knee OA. The annual lifetime health care burden (i.e. the total cost of treatment per year over the lifetime) ranges from $7.6 billion for those who opt to undergo surgical reconstruction and $17.7 billion who opt for non-surgical rehabilitation.

One of the major hurdles patients face after ACL injury and reconstruction is quadriceps dysfunction. The quadriceps plays a critical role in absorbing impact forces during tasks such as walking and running. When the quadriceps doesn’t work properly, their ability to absorb these impact forces is diminished, and cartilage in the joint experiences greater loading. Given the repetitive nature of walking and its integral role in human locomotion (≥ 10,000 steps per day), this greater loading contributes to breakdown of articular cartilage and development of post-traumatic knee OA (Figure 1).

Quadriceps dysfunction is caused by a complex neuromuscular phenomenon called arthrogenic muscle inhibition (AMI). In short, sensory signals from the knee joint inform the central nervous system that injury has occurred, and the nervous system responds by inhibiting the quadriceps to prevent further loading of the injured tissues during the acute phase of recovery from the injury. However, AMI persists well past the acute phase, as quadriceps dysfunction has been documented more than a decade following ACL injury. This lingering quadriceps dysfunction is thought to contribute to exaggerated cartilage loading and development of knee OA.

Though strengthening the quadriceps is an integral part of ACL rehabilitation, it is often ineffective due to the fact that it doesn’t address AMI. Introducing modalities that first reduce AMI prior to strengthening exercises may improve rehabilitation. Both direct (local muscle vibration – LMV) and indirect (whole body vibration – WBV) vibratory stimuli enhance muscle function, and may minimize the negative effects of AMI (Figure 1). Therefore, the purpose of this study was to evaluate the effects of WBV and LMV on quadriceps function in individuals with simulated knee joint pathology.

What did you do and what did you find in this study?

Quadriceps function was measured during maximal contractions (Figure 2) via the central activation ratio (CAR). CAR quantifies an individual’s ability to voluntarily activate a muscle, and is an indicator of AMI. During maximal contraction, an electrical stimulus is applied to the muscle which produces an increase in torque. CAR refers to the ratio of maximal voluntary torque to the torque resulting from the electrical stimulus (a vs. b in Figure 3).

Quadriceps AMI can be produced experimentally by injecting saline in the knee joint, which mimics swelling (i.e. simulated joint injury). Following pre-test measures, 60mL of saline were injected into the knee joint to induce AMI. Subjects were then exposed to WBV, LMV, or Control (no vibration) interventions (Figure 4), and quadriceps function was then reassessed post-intervention. CAR improved significantly with WBV (+8.3%) and LMV (+5.9%), but not in the Control group (+0.7), and this improvement did not differ between LMV and WBV (Figure 5). Additionally, subjects who experienced the greatest amount of AMI displayed the greatest improvements in quadriceps function with vibratory stimuli.

How do these findings impact the public?

These findings suggest that vibratory stimuli may be an effective tool for improving rehabilitation of ACL injury and other knee pathologies by reducing AMI and enhancing quadriceps function. This enhancement of quadriceps function may, in turn, reduce the risk of developing knee OA, a disease which reduces quality of life, is a leading cause of disability, and contributes to multiple comorbidities such as obesity and depression. Given the immense economic burden of knee OA on the US health care system (> $50 billion per year), these findings could have substantial relevance for reducing health care costs and disability.